The present invention relates to a focusing circuit for generating a focus control signal for a cathode ray tube, the focusing circuit having a modulation input for influencing the focus control signal, which focus control signal contains a measure for an envelope of excursions of horizontal scan lines across the cathode ray tube. The present invention also relates to a cathode ray tube and an image device, such as a monitor or television provided respectively with such a focusing circuit.
EP-A-0 546 595 discloses a circuit for focusing an electron beam generated in a cathode ray tube (CRT) having a television display screen. The known focusing circuit comprises a diode modulator circuit for generating a generally parabolic dynamic horizontal focus voltage control signal. The control signal is dynamic in that a voltage that varies with the angle of deflection of the horizontal electron beam is added to a static focus voltage applied to said beam. It is parabolic in shape because the magnitude of the voltage has to increasexe2x80x94due to a defocusing effect emanating from the cathode ray tubexe2x80x94in order to fully cover focused east and west edges of the screen in particular at the north and south parts thereof. This defocusing effect can be corrected.
There is however a second effect known as the pin cushion effect. This is the effectxe2x80x94as also described in the prior art abovexe2x80x94that without correction the scan lines in the center of the screen are shorter in length than the scan lines in the north and south parts of the screen. In order to compensate for the pin cushion effect the horizontal deflection current of a scan line is made depend on its distance to the center of the screen. If however the dynamic focus control signal is derived from the deflection current such as by integration over time, a combined effect occurs and the control signal is modulated undesirably. In the mentioned prior art this undesirable modulation in the focus control signal is magnetically compensated in a specially devised transformer.
However the prior art compensation does not work satisfactory for a cathode ray tube which has to be capable of working in multiple different display monitor and/or television modes used at present, such as Hercules, MONO, EGA, CGA, VGA, S-VGA, PAL, HDTV etcetera having different resolutions, line numbers and line rates, and all requiring a specific focus control signal modulation.
Therefore it is an object of the present invention to provide a focusing circuit which has a broadened application area, and which is capable of at least partly compensating for said negative effects in cathode ray tubes capable of working in plural different modes.
Thereto the focusing circuit according to the invention is characterized in that the focusing circuit comprises a load control means which is embodied for influencing the focus control signal, and that the load control means have a control input coupled to the modulation input for influencing the focus control signal by load control variation in dependence on a desired focus control signal modulation.
It is an advantage of the focusing circuit according to the present invention that it consumes a limited amount of power and that by connecting an easy to implement load control means to the uncorrected and uncompensated focus control signal, at wish both aforementioned effects in a cathode ray tube working in any kind of mode can be compensated, without bulky and specially manufactured and devised means, such as a transformer. The desired influence for the modulation input can simply be taken from a synchronization or deflection processor present in any image device, such as monitor or television. Advantageously the functioning of the focusing circuit according to the present invention is independent of the graphics mode applied on a monitor device or television device. Thus the working area of such devices is no longer restricted since these devices are now capable of working properly in any graphics monitor or television mode desired. In addition the focus control no longer depends on the way the negative effects mentioned above are being compensated. Furthermore a large degree of freedom regarding the control of the horizontal parabola is possible which gives way to additional design freedom in the focusing circuit and image device according to the invention.
An embodiment of the focusing circuit according to the invention is characterized in that the load control means comprises a current source and/or a voltage source and/or a controllable impedance that have a control input for coupling to a modulation output of a synchronization processor.
It is an advantage of the focusing circuit according to the invention that at wish a controllable current and/or voltage and/or controllable impedance can be used in the load control circuit to control the focus control signal adequately.
A further embodiment of the focusing circuit according to the invention is characterized in that the controllable impedance comprises an inductive and/or capacitive and/or semiconductor circuit coupled to any of said respective sources.
Advantageously in a further embodiment of the focusing circuit according to the present invention the impedance may be provided with readily available components such as capacitance components and/or inductance components and/or semiconductor components.
A following cost effective and easy to manufacture embodiment of the focusing circuit according to the invention is characterized in that the controllable impedance is embodied by at least one controllable semiconductor.
A still further embodiment of the focusing circuit according to the invention is characterized in that the controllable impedance comprises a series arrangement of two capacitors, and a field effect transistor having a main current path coupled to the connection point of said capacitors, and a control input coupled to the modulation output of the synchronization processor. This embodiment though effective only comprises a minimum number of components.
If necessary for reversing the phase of the modulation control signal a reverse circuit is coupled between the synchronization processor and the control input of the load control means.